Field of the Invention
The present invention relates to a communication apparatus and a communication system that are utilized for assisting a driver in his drive and control of a vehicle through inter-vehicle communication, which is communication between a vehicle and another vehicle.
Description of the Related Art
In a drive assistance system and a vehicle control system, vehicles exchange information items such as traffic situations and the positions of respective reference vehicles through inter-vehicle communication, for the purpose of preventing a traffic accident and alleviating traffic congestion.
In this case, in general, the vehicle of an information provider broadcasts (including “multicast”, and the same applies hereinafter) the same information with a maximum transmission power and in a recurrent manner, for the purpose of securely providing the same information to unspecified communication counterparts around the vehicle.
However, communication with a constant maximum transmission power among vehicles causes a problem because in an environment where the traffic is heavy, electromagnetic-wave interference is liable to deteriorate the qualities and the efficiencies of respective communication instances.
As a method of controlling transmission power for the broadcast, for example, the method disclosed in Patent Document 1 is known. In this method, in order to raise the communication quality, a data reception vehicle demands that communication power should be increased when the communication quality is the same as or lower than a specified value, and hence a data transmission vehicle increases the communication power and then transmits the data again.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-283733
However, in the control method disclosed in Patent Document 1, the emergency degree or the like of data contents themselves is not considered; therefore, for example, there has been a problem that because two or more vehicles resend low-emergency-degree data (e.g., the road traffic information of a 10-km-apart position) with increased transmission power, high-emergency-degree data (e.g., information of a vehicle that is approaching) from another vehicle cannot be received.